The invention concerns an apparatus for the ignition of a fuel/air mixture in the combustion chamber of a combustion machine, wherein the combustion chamber has at least one inlet valve and at least one outlet valve. There are further provided a laser light generating device for giving off laser light and a combustion chamber window for coupling the laser light into a combustion chamber of the combustion machine. The invention further concerns a method of operating a combustion machine, in particular a gas engine, using a laser light generating device which introduces laser light into a combustion chamber of the combustion machine, wherein the laser light generating device has a combustion chamber window by way of which the laser light is introduced into the combustion chamber. Finally the invention concerns a combustion machine having an apparatus of the aforementioned kind.
Laser ignition is an ignition system which is in the course of development for combustion machines operated on the basis of the Otto cycle, which is based on the principle that an intensive laser pulse in the combustion chamber of the combustion machine is concentrated on to a focal point, whereby the extremely high field strengths of the laser light beam which occur in the focal point or at the focus cause the gas to be ionised and consequently heated to plasma temperatures (several thousand degrees Kelvin). Due to focusing of the laser light beam on the focal point, the ignition spark or sparks is or are generated there. The resulting plasma spark ignites the fuel/air mixture in a similar manner to conventional spark ignition in which the spark is produced by electrical flash-over between two electrodes.
There are different concepts for producing the laser light pulse. A preferred concept provides that the ignition laser which produces the ignition pulse is longitudinally pumped by means of a pump light source (for example a semiconductor laser) by way of an optical fiber until the activation energy reaches a level required for starting and delivery of the ignition laser pulse. The beam of the pulse laser is introduced into the combustion chamber by way of a suitable optical means comprising a focusing device and a transmission window (combustion chamber window). The optical coupling-in means for coupling the laser pulse into the combustion chamber of the engine comprises a suitable lens system and what is referred to as the combustion chamber window representing the last optical element before the beam passes into the combustion chamber.
The advantage of laser ignition over conventional spark ignition is inter alia that the ignition spark can be placed freely in the depth of the combustion chamber where optimum ignition conditions prevail. In contrast thereto, combustion initiation with conventional spark ignition occurs in the immediate proximity of the combustion chamber wall, wherein the flat electrodes delimiting the ignition spark impede formation of the flame core. The energy of the laser spark can be greatly increased by increasing the power of the laser system without thereby involving increased wear as occurs with spark ignition in regard to electrode wear.
A further advantage of laser ignition is that with increasing engine power output the minimum pulse energy required (which is that energy of the plasma spark, that is required at a minimum to ignite the fuel/air mixture) decreases. In comparison, the conventional spark ignition systems with the engine power outputs planned in future noticeably encounter their system limits. Particularly in the case of large-scale static engines, preferably gas engines which in the present case represent a preferred area of use, ongoing use both of the engine and also the ignition apparatus must be possible, with long service lives, in order to keep stoppage times (for example, for replacing ignition systems) as short as possible.
The major problems in terms of designing and mass-production implementation of laser ignition include inter alia ensuring or maintaining the optical properties of the combustion chamber window over the service life of the combustion machine. Especially in relation to the combustion chamber-side interface of the combustion chamber window, high thermo-chemical loadings and the deposit of solid residues from combustion can lead to clouding of the surface, whereby the beam is attenuated, (that is to say partially absorbed and also scattered), which either leads to a considerable reduction in the energy of the plasma spark or leads to failure of the plasma spark.
That problem is usually combated by on the one hand providing reserves for losses and attenuation effects due to the service life by means of high levels of pulse energy and on the other hand producing the high levels of pulse power to afford an effect of burning the window surface free. The disadvantage of that procedure lies in the considerable increase in costs for the required high laser power output and in the high specific loading on the surface at which the window is burnt free.